1. Field of the Invention
The present invention relates to a full color image forming apparatus (such as a copying machine, and a printer, for example) with a plurality of photosensitive members.
2. Description of the Related Art
In recent years, a configuration in which image forming units for each color are arranged on intermediate transfer members as illustrated in FIG. 9 (hereinafter referred to as an intermediate transfer tandem method) has been used according as a full color image forming apparatus using an electrophotographic method has been increased in speed and materials have been diversified. Cylindrical intermediate transfer drums and an intermediate transfer belt stretched between a plurality of rollers are used as the intermediate transfer member. FIG. 9 illustrates the configuration using the latter being high in the degree of freedom of a stretching layout.
An image forming apparatus 60 illustrated in FIG. 9 is configured such that a full-color toner image formed on an intermediate transfer belt 605 is transferred on a recording material S conveyed from a recoding material storage unit 62 at a transfer unit formed of a secondary transfer inner roller 608 and a secondary transfer outer roller 66. The full-color toner image is formed in order from a yellow (hereinafter referred to as Y) image forming unit positioned in the most upstream side with respect to a direction indicated by an arrow B. In other words, an image forming process for magenta, cyan, and black (hereinafter referred to as M, C, and Bk respectively) is sequentially started at predetermined timing intervals. The toner images M, C, Bk are superposed on the toner image Y on the intermediate transfer belt 605. The toner image Y located in the most upstream position is taken as an example to describe a basic image forming process. The photosensitive member 611Y which serves as an image carrier and rotates in a direction indicated by an arrow D is uniformly charged by a primary charging device 612Y. An electrostatic latent image is formed by image exposure of an exposure apparatus 609Y based on image information previously formed on the charged photosensitive member 611Y. Light from the exposure apparatus 609Y reaches an exposure position on the photosensitive member 611Y through a diffractive member 610Y. The electrostatic latent image is visualized as a toner image by a development device 613Y and electrostatically transferred onto the intermediate transfer belt 605 by a primary transfer roller 618Y as described above. The similar processing is executed in other image forming units.
Thus, a plurality of the image forming units is arranged side by side to increase a distance of movement of an image. In other words, a substantial distance of movement of an image is equal to the sum of a perimeter (in the direction D) of the photosensitive member 611Y from the exposure position in the image forming unit Y located in the most upstream position to a nip portion of the primary transfer roller 618Y and a perimeter (in the direction B) of the intermediate transfer belt 605 from the nip portion of the primary transfer roller 618Y to the secondary transfer unit. Such a long distance of movement of an image is relatively apt to exceed a distance of movement of the recording material S (i.e., a conveyance distance between a paper feeding apparatus 63 and the secondary transfer unit).
When the distance of movement of an image is longer than the distance of movement of the recording material to the secondary transfer unit, the timing at which an image is started to be formed needs to be earlier than the timing at which the recording material is started to be conveyed.
As a result, if any failure in paper feeding, such as a failure to pick up paper, occurs in the paper feeding apparatus 63, a complicated control is required to match the timing between the image and the paper in the control for starting feeding paper after starting forming an image. For this reason, the time, during which the image is started to be formed and then the toner image reaches the secondary transfer unit, is desirably shortened as much as possible.
Japanese Patent Application Laid-Open No. 2006-301332 discusses a configuration in which a plurality of secondary transfer units is provided to shorten the time during which an image is started to be formed and then a toner image reaches the secondary transfer unit. The configuration discussed in Japanese Patent Application Laid-Open No. 2006-301332 is provided with two intermediate transfer belts 73a and 73b for four photosensitive members 70a to 70d as illustrated in FIG. 10A. Further, there are provided two secondary transfer nips Ta and Tb in which a secondary transfer belt 74 is backed up by secondary transfer rollers 75 and 76. In this configuration, as illustrated in FIG. 10A, a first and a second color are transferred from the photosensitive members 70b and 70a onto the intermediate transfer belt 73a by primary transfer rollers 71b and 71b. On the other hand, a third and a fourth color are transferred from the photosensitive members 70c and 70d onto the intermediate transfer belt 73b by primary transfer rollers 72c and 72d. This produces an effect of shortening the distance of movement of an image.
As illustrated in FIG. 10B, the image forming unit is completely divided into two portions to allow obtaining an effect of further shortening the distance of movement of an image using an image process equivalent to the above described pattern in FIG. 10A. Thus, providing a plurality of secondary transfer units allows shortening the distance between the exposure position of the first color and the transfer unit where a toner image is transferred on a recording material. Accordingly, time during which an image is started to be formed and then a toner image reaches the secondary transfer unit can be shortened.
Japanese Patent Application Laid-Open No. 2006-301332, however, has the following problem because it needs a plurality of the secondary transfer units, i.e., multiple transfers onto the recording material. More specifically, the secondary transfer unit in a second time is affected by the toner image transferred by the secondary transfer unit in a first time to make the transfer condition strict.
For that reason, the number of times that a toner image is transferred on a recording material is desirably made as smaller as possible in forming a full-color image.